Sensitized photooxygenation VI. Kinetic studies on the singlet oxygenation of 6‐ethyl‐3, 4‐dihydro‐2H‐pyran‐5‐carboxylic acid ethyl ester and 6‐isopropyl‐3,4‐dihydro‐2H‐pyran‐5‐carboxylic acid ethyl ester

Abstract

AbstractKinetic studies of the singlet oxygenation of the title compounds were performed according to Monroe's method. The reaction rate increases with temperature decreasing, leading to a negative activation enthalpy and a large negative activation entropy. These data are interpreted as the evidence for the intermediacy of an exciplex. The solvent effect on the reaction rate suggests that the “dioxetane” path involves a transition state or an intermediate with significant zwitterionic character. The electronic effect of the substituent is obvious, with electron‐withdrawing substituent retarding the reaction and electron‐donating substituent increasing the reaction rate. However, steric bulkiness at the 6‐position does not play an important role in the reaction rate.